Reading a PSD, can I get a mean frequency from this plot? 3

[Mahti86]

Hi!

I´ve tried to analyze the plot attached for a while now, but it does´nt make any sense to me. I´ve never encountered a PSD plot before and I need a frequency (mean) to continue my calculation on vibrations, but the PSD just makes me confused. Could anyone extract some information from it to me?

Acceleration data:

http://files.engineering.com/getfil...-4199-8f97-4c6b72b2fc37&file=Eng_help-Acc.jpg

PSD for the acceleration data:

http://files.engineering.com/getfil...4cfe-4e34-a257-9859aaa33e34&file=Eng_help.jpg

Kindly //

 

[GregLocock]

I concur, that you cannot usefully derive a frequency(mean) from that plot.

I suggest you go back to whoeever asked you for a better definition. It is not an expression i have come across in 30 years of vibration analysis.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Mahti86]

Thanks for your reply Greg. You just saved me a whole bunch of time

 

[Twoballcane]

Are you asking for the resonate frequency? I've never heard of mean (average?) frequency.

Tobalcane
"If you avoid failure, you also avoid success."
“Luck is where preparation meets opportunity”

 

[Mahti86]

A resonate frequency would be perfect, if the psd provides the information needed

 

[Twoballcane]

If your input PSD is as flat as NAVMAT-P-9492 with a broader band (50Hz to 1000Hz at 5dB down), the Fn would be the first highest peak from left to right. If the profile looks like a Harley Davidson engine, it is very difficult to determine the Fn. However, a good tester would do a .25g sin sweep first to tease out the Fn so that when you do the random vibe, you can compare the energy imparted into the part due to the random vibe at that frequency. If anything, ask if they can do a sin sweep.

Tobalcane
"If you avoid failure, you also avoid success."
“Luck is where preparation meets opportunity”

 

[BrianE22]

From the units used in the PSD it looks to me like a PSD was taken of the (time history - avg. accel of the time history). Your time history does seem to have an offset. Maybe someone thought it was best to get rid of the offset by subtacting the average acceleration from the time history before taking the PSD.

 

[amanuensis]

To carry out a PSD, you have to suppose that your signal is stationary and ergodic (it is the definition of a random signal)

Your time-history accel. is not enough flat ( many peaks).
In my opinion, your signal is not enough stationary to perform a PSD...

 

[IRstuff]

Is there more to the PSD plot? It appears to be falling off past 100Hz. That might indicate a corner frequency.

What exactly are you trying to calculate with your "mean" frequency? The only thing I can imagine using a single frequency is to determine whether a particular mechanical frequency overlays on top of an isolator frequency, which is undesirable.

TTFN
faq731-376

 

[Mahti86]

Thank you for your help, and sorry for the late answer - everyone needs vacation

The plot is from a test vehicle, and I need the frequency to complete some calculations within piezoelectrics.

 

[IRstuff]

Again, what is the "mean frequency" supposed to represent?

TTFN
faq731-376

 

[Mahti86]

IRstuff, Im a bit lost in my calculations of piezoelectricity. But hopefully Im a bit more educated within the area now
What Im looking for in terms of frequency is the bandwidth where the most of the frequency is. Since I need to order piezoelements that work in that specific frequency range.

Is that easier to outline from the PSD? Thanks in advance (again!)

 

[IRstuff]

I still don't understand. The standard bandwidth is essentially defined by the intersection of the DC response level vs. the sensor rolloff, which is a straight line with a 10dB/decade slope for power.

TTFN
faq731-376

 

[Mahti86]

Ok, now Im completely lost :/

The only data I have is the acceleration graph and the PSD for it. They have been measured by someone else, and was given to me since Im doing studies that require an operating frequency. I need to pinpoint a frequency from which I could harvest the most energy from (piezoelectrics), and I have no idea how to read the PSD and what the strange unit (m/s2-rms^2)/Hz tells me.

Sorry, but my knowledge within electronics and vibrations are highly limited..

 

[IRstuff]

Perhaps, rather than getting mired in a subject you appear to know nothing about, you should simply describe what end result you are trying to achieve.

It seems to me, reading between the lines, you are actually needing to use the amplitude for a particular frequency to determine the amount of power your "piezoelectrics" produce? You then integrate with respect to frequency, and divide by the average bandwidth.

You do have a problem in that your PSDs are not rolling off, so there's no information about where the input energy actually starts to fall off.

TTFN
faq731-376

 

[BrianE22]

A convential PSD does tell you where most of your power or energy is. The units on yours are a bit wierd. I think the PSD was taken from a time history for which the RMS of the time history was subracted (my guess based on the units). The DC value and the low frequencies (< 5 hertz) in your data are the most dominant but they may not be real. Your PSD is fairly flat with no real dominant frequencies. There's a couple of peaks around 16 hertz and 85 hertz. I'd think you'd want a piezo device with a flat response through 100 hertz minimum. If you want to go down to DC that will determine the low end of the response.

 

[Mahti86]

IRstuff: I do want to determine the power I could achieve from a PZT, bi- or unimorph plates mounted within a vechicle. With the hopes of harvesting the energy from ambient vibrations. Actually the amplitude would probably have been a issue after I managed to get my head around the frequency mystery. Average bandwidth might be the key-word I´ve been looking for?
I do actually have one more PSD from another measurement. I´ll post it here as soon as I get into the office tomorrow.


BrianE22: I tried to get hold of the guy who passed along the PSDs for me on order to see if they have other PSDs to get my hands on. I´ve checked other information graphs and I think the frequencies should be around 20Hz. Could the two peaks be about resonance frequencies?

Flat response through 100Hz minimum? And the circuitry I´ve planned on following uses a simple rectifying bridge to get DC, how does this determine the low end of the response?

ps. I really appreciate your time & thanks for the responses even though my replies tend to be a bit slow. Different time zones makes it hard for me to answer right away.. I´ll post the other PSD tomorrow

 

[IRstuff]

Well, you are missing a huge part of the problem, from what I can tell. The PSD is only marginally useful for what you're trying to do, since you do not have anything resembling the transfer function between the vibration source and the piezoelectric device.

TTFN
faq731-376

 

[Mahti86]

Do you mean a physical transfer function, or an equation? I have a few articles available with equations for calculating the piezoelectric power with dependence of the vibrations from the source.
That´s the reason Im doing this, I can´t order any piezoelectric devices if I don´t have anything pointing out that I actually can get somewhere close the power output I need (with current being the limiting one).


Here is another graph with acceleration data and the PSD for it:

 

[IRstuff]

Things like this are highly application, design, and location dependent. I don't see how it would be possible to predict anything without a solid, physics-based analytical model. Piezoelectricity is dependent on the applied stresses across the device. If is completely rigid body motion, then there would be no output.

Conversely, your "current" requirement may simply exclude this approach completely. Piezoelectric devices are not high-current sources.

TTFN
faq731-376